Rigid polyurethane foams are widely known and are used in numerous industries. Rigid foams can be used, for example, in the construction industry as insulating materials as well as in the manufacture of appliances. Rigid polyurethane foams are produced by reacting a polyisocyanate with a polyol in the presence of a blowing agent. Chlorofluorocarbons (CFC's) were typically used as blowing agents in the production of rigid polyurethane foams. When used in the production of rigid polyurethane foams, CFC's vaporize as a result of the reaction exotherm, thereby causing the reaction mixture to form a foam. CFC's are now believed to contribute to the depletion of ozone in the stratosphere. As a result, mandates have been issued which prohibit the use of CFC's. Effort has therefore been directed to using water as a blowing agent in the production of some rigid polyurethane foams.
There are, however, drawbacks to using water as a blowing agent for producing rigid polyurethane foams. One such drawback is the fact that carbon dioxide gas, which is produced from the reaction between the isocyanate and water, tends to diffuse out of the foam cells, thereby leading to foam shrinkage. As more water is used in production processes, the resulting foams become less dimensionally stable, i.e., the foams shrink upon storage. It is generally recognized in the art that water-blown rigid foams having densities less than 2.4 lbs/ft3 are not dimensionally stable, particularly at elevated temperatures. (See U.S. Pat. No. 5,420,169, column 2, line 25).
In an attempt to alleviate the foregoing problem, several approaches have been taken. One such approach is to produce open-cell rigid polyurethane foams. However, unlike closed-cell water blown foams, open-cell foams are poor insulators. (See W. A. Kaplan et al., Low-Density All Water-Blown Rigid Foam for Pour-in-Place Applications, Polyurethanes Expo '96 Conference Proceedings, pp.179-89 (1996)). Alternatively, highly functional, low molecular weight polyols can be used in the production of low density water-blown rigid foams to increase crosslinking density and improve foam strength and reduce shrinkage. However, these foams are generally brittle and tend to have poor adhesion to substrates.
A process for producing a dimensionally stable, low-density water-blown rigid foam is described in U.S. Pat. No. 5,627,221. The foam produced by the process described in this patent has a free rise density less than about 2.5 lbs/ft3 and exhibits less than a 5.5% volume change when stored at 158° F. for twenty-eight days. The polyol mixture used in the process described in this patent is composed of about 50-90 parts by weight of a polyester polyol and about 5-50 parts by weight of a polyether polyol. However, the foams produced by the process disclosed in this patent have low compressive strength. (See Example 1, column 5, lines 51-52).
There remains, therefore, a need for dimensionally stable, low density water-blown rigid foams which have acceptable compressive strength.